U.S. Pat. No. 4,884,008 and corresponding German DE-OS 3723009 disclose an operating theater lamp with multiple light beams projecting from the bottom on the lamp housing. These beams are aligned in such a way that they overlap in the operating field and form an intensified illumination field. The alignment is done with the aid of an ultrasonic distance meter located in the fixture. The meter generates a signal representing the distance between fixture and operating field, and this signal is fed to a control circuit as a command or guidance value. In the control circuit, this guidance value is compared with the angle setting of the light beam which serves as the control value. In the event of deviation or difference between these two values, an adjusting signal is applied until the control value agrees with the guidance value. The associated electrical circuit inhibits the control circuit from reacting to passage, through the light beam, of suddenly appearing measurement targets, e.g. the hands or head of the surgeon. The release of the beam adjustment mechanism therefore occurs either by manual actuation of a handle located on the lamp housing or by sensed increase in distance, such as occurs upon deepening of the operating incision. In this way, one can achieve an automatic, trouble-free beaming or direction of the light, without special equipment.
Further, MENKE DE-OS 32 27 494 discloses an operating lamp, for dental and oral surgery procedures, in which the light beam is kept automatically aligned on the mouth area of the patient throughout adjustments to the dental chair or couch. See discussion at U.S. Pat. No. 4,884,008, col. 2, lines 10-30. The associated tracking mechanism uses an ultrasound transmitter 32 located on or near the patient's head, ultrasound receivers 38 spatially separated around the treatment room, and a direction finding circuit 40. The tracking is done using servo- or stepper-motors by which the holding of the lamp in specified positions or inclinations can be carried out. Due to the need to locate an ultrasound transmitter in the immediate vicinity of the operating field to be illuminated, such an operating lamp is unusable in general surgery. In particular, one would have to expect errors in adjustment, as well as difficulties in handling and sterilization.